Conventional dowels for mounting insulating plates have the disadvantage, that they cannot entirely prevent the formation of thermal bridges, which also applies for dowels having a plastic coat or heads made of plastic. Substantially, this results from the diversity of the materials of which the insulating material on the one hand and the dowel on the other hand are made. This diversity may also lead to undesirable dowel show-through in the plaster layer at the outer walls, especially when exposed to moisture and wetness. This disadvantage occurs frequently when applying basic plaster qualities. To achieve a better thermal insulation and to avoid dowel show-through in the plaster layer, a sunk-in mounting of the dowel in the insulating plate has been suggested.
A process where a recess is formed in the insulating plate for taking up the dowel, simultaneously with the drilling of the bore-hole for the dowel is known from EP 0 086 452. In this case, a recess corresponding to the diameter of the pressing plate is milled into the insulating plate when drilling the bore-hole in the same process step, by means of a countersink drill which includes a milling head and a plate-like stopper, and subsequently closed with a covering which is already arranged on said pressing plate. Due to the fact that the covering is arranged on the pressing plate respectively on the dowel head as provided by the factory, an additional process step is avoided during mounting. However, the milling leads to a considerable abrasion of insulating material. Considering the large number of dowels which are regularly used for mounting insulating plates when building a house, the generated milling dust poses a considerable pollution of the environment, and, in larger amounts, may also lead to health problems for the construction personnel. Further, the mounting properties of dowels having coverings applied at the factory are clearly inferior to comparable dowels without applied covering. Due to the insufficient resistance to pressure of the insulating material and the thus limited drive-in-forces, only low dowel expansion forces are possible, which can lead to considerably poorer values regarding the draw-out resistance. Further, in production such dowels are significantly more expensive than conventional embodiments. Additionally, for carrying out the known process a specific countersink drill is required, being likewise expensive in production.
A further known solution comprises a milling crown, e.g. made of plastic, by means of which a recess can be milled into the insulating material after the bore-hole has been drilled. Then the dowel is introduced into the bore-hole and subsequently mounted. Thereafter, the covering is applied. This solution contains two more process steps as compared to EP 0 086 452 and therefore is more time-consuming. Apart from that, there is also a considerable environmental burden because of the generated milling dust. EP 1 088 945 discloses an insulating element, which consists of mineral fibers and comprises markings for the arrangement of dowels for insulating material. The markings can be located in recesses within the insulating element respectively be formed as recesses. Preferably, the recesses are formed mechanically or in the case of thermoplastic insulating materials by local heating. The production of such insulating elements is connected with considerable costs on the one hand, on the other hand the pre-fabricated recesses leave no room for the construction personnel on-side for an assembly of the dowel for insulating material which deviates from the predetermined pattern. It is also not taken into consideration that the respectively most advantageous geometric arrangement of the dowels for insulating material with regard to the insulating plate may vary enormously depending on the special circumstances and the weather conditions.
Further, pre-forms of mineral wool for acoustic and thermal insulation of household appliances are known from utility model DE 296 22 196. These form-parts comprise areas of material, in which recesses are formed. These areas of material are thereby exposed across the depths of the predetermined recess by cut-ins with regard to the surrounding material, and are already pre-compressed during the production by means of a pressure piston. Thereby, the material is compressed in such a way that it comprises a residual elasticity for enabling a limited adaptation to changes in the configuration during the assembly of ovens etc. Also in this case, expensive pre-formed parts are considered. For the construction industry, especially with regard to the mounting with dowels for insulating material, parts produced in such a way are not suitable since they do not comprise the required stiffness.